MOSCOW: Russian opposition leader Boris Nemtsov, a fierce critic of President Vladimir Putin, was gunned down while walking in sight of the Kremlin late Friday, prompting an international chorus of condemnation.

The Kremlin decried as a provocation the murder of the 55-year-old former deputy prime minister, which came ahead of an opposition rally he was set to lead this weekend and sent shock waves across the country.

US President Barack Obama led condemnation of the “brutal” and “vicious murder” of Nemtsov, whom he had met on a visit to Moscow, and called on Russia to conduct an impartial probe.

Putin's spokesman Dmitry Peskov said the Russian leader had taken the investigation “under personal control”.

The murder of the outspoken critic of Russia's involvement in Ukraine, on a bridge near the Kremlin, “bears the hallmarks of a contract killing,” Peskov said on radio, describing it as a provocation.

The brazen assassination was one of the most highest-profile killings during Putin's 15 years in power and recalled the shooting of anti-Kremlin reporter Anna Politkovskaya, who was gunned down on Putin's birthday in 2006.

Investigators said Nemtsov was shot by unidentified assailants as he was walking with a Ukrainian woman along a bridge just metres from the Kremlin.

“According to preliminary information, an unidentified person shot at Boris Nemtsov no fewer than 7-8 times from a car as he was walking along the Bolshoi Moskvoretsky bridge,” said investigators.

Interior ministry spokeswoman Yelena Alekseyeva said Nemtsov was hit by four bullets.

An AFP reporter saw a blood stain on the rain-soaked pavement on the side of the bridge near the red walls of the Kremlin, with roses laid by a police barrier at the scene.

'Beyond imagination'

Speaking on radio just hours before his murder, Nemtsov sounded upbeat and urged Russians to take to the streets and join a major opposition rally planned for Sunday.

“The key political demand is an immediate end to the Ukraine war,” he said on popular Echo of Moscow radio, adding that Putin should quit.

“A dead-end in both domestic and foreign policies. They should go,” said Nemtsov, who reportedly was working on a report detailing Russia's involvement in the Ukraine conflict.

After working as a research scientist in the late Soviet era, Nemtsov rose to prominence as governor of the Nizhny Novgorod region in central Russia and became a vice prime minister in the late 1990s under the presidency of Boris Yeltsin.

After leaving parliament in 2003, he led several opposition parties and groups.

An accomplished orator with a rock star image and popular with women, Nemtsov was one of the key speakers at mass opposition rallies against Putin's return to the Kremlin in 2012.

He developed a reputation for writing critical reports about corruption and other misspending under Putin.

In 2013, he said up to $30 billion of the estimated $50 billion assigned to the Olympic Games Russia was to host in Sochi had gone missing.

The Kremlin has denied the claims.

“This is payback for the fact that Boris consistently, for many, many years fought for Russia to be a free democratic country,” opposition leader Mikhail Kasyanov, who served as prime minister under Putin, told reporters after viewing the scene of the murder.

“In the 21st century, in 2015, a leader of the opposition is shot dead by the Kremlin walls. It is beyond imagination.”

'Tireless advocate'

Washington was among the first to condemn the killing.

“We call upon the Russian government to conduct a prompt, impartial, and transparent investigation into the circumstances of his murder and ensure that those responsible for this vicious killing are brought to justice,” Obama said in a statement.

“Nemtsov was a tireless advocate for his country, seeking for his fellow Russian citizens the rights to which all people are entitled.”

Ukrainian President Petro Poroshenko, writing on Facebook, called Nemtsov a “bridge between Ukraine and Russia”.

“The murderers' shot has destroyed it. I think it is not by accident.” Thorbjorn Jagland, secretary general of the Council of Europe, added he was “shocked and appalled. “ “Killers must be brought to justice,” he said on Twitter while London said it was “shocked and saddened”.

“We deplore this criminal act. Those responsible must be brought to justice,” a Foreign Office spokeswoman said.

Human Rights Watch also urged the Russian authorities to “thoroughly and impartially” investigate the murder.

'New political reality'

Opposition activists and ordinary liberal-minded Russians went into mourning, posting tributes to Nemtsov on social networks throughout the night.

Activists had scrapped plans to hold the Sunday opposition rally in southeast Moscow and planned to conduct a funeral march in the city centre instead.

“We are in a new political reality,” one of the organisers, Leonid Volkov, said on Twitter.

Exiled opposition leader Mikhail Khodorkovsky said his family was grieving.

Alexei Venediktov, editor-in-chief of Echo of Moscow radio, wrote that Nemtsov, who leaves behind four children and an elderly mother, had known he had taken risks by openly criticising Putin.

“But I will not leave Russia, who would fight then?,” he quoted the veteran politician as saying.